Abstract
MnAl alloys are considered to be an important candidate to plug the gap between Nd-Fe-B magnets and ferrites, and their microstructure is closely related with the magnetic properties. In this work, the relation between solidification microstructure and the coercivity as well as the ε→τ solid-state transformation in MnAl permanent-magnet alloys are investigated. Different solidification microstructures are obtained in (Mn0·54Al0.46)98C2 alloys by various preparation methods. The solidification microstructure undergoes a significant refinement from single crystal to columnar grains, dendritic grains and finally to ultrafine columnar grains by increasing the solidification rate, and the width of the grains monotonously decreases from ∼3 mm for single crystal finally to ∼2 μm for the ultrafine columnar grains. Besides, the ε→τ transformation temperature and coercivity of τ-phase have significant correlation with the initial microstructure of ε-phase. The ε→τ solid-state transformation gradually shifts to lower temperatures due to the nucleation stimulation by higher density of grain boundaries with the refinement of solidification microstructure. Simultaneously, the coercivity is remarkably improved from 0.16 kOe in single crystal to 1.47 kOe in melt spun ribbons with ultrafine columnar grains because of the pinning effect of the increasing amount of grain boundaries.
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